The present invention relates to cathode cups and electrodes for x-ray tubes. In particular, the invention relates to cathode cups for x-ray tubes, and a method of manufacture of cathode cups.
In high voltage electron beam tubes such as, for example, x-ray tubes, an electron emitter (typically a filament situated in a cathode cup) is caused to emit electrons by heating and simultaneous application of a high voltage thereto. Electron emission may be controlled by varying the filament current and thus the temperature of the filament. Also electron emission can be controlled by use of a "grid" voltage whose potential with respect to the electron emitter, such as but not limited to, a filament may be varied to either accelerate or retard a beam of electrons emitted by the filament. In a x-ray tube, the cathode may comprise a filament, which is heated by electrical current to emit electrons, and a surrounding cathode cup, which acts to focus electrons emitted by the filament, and possibly to act as a grid control. The cathode cup generally has one or more slots within which the filament, typically a helical coil of tungsten wire, is positioned.
In order to release electrons, the metallic filament that is positioned in the cathode cup is electrically heated to incandescence for example by means of the passage of a predetermined current therethrough the current causes thermionic emission of electrons. The released electrons are accelerated by the application of a high voltage between the cathode and the anode of the x-ray tube. Impingement of the accelerated electrons upon a target anode, typically in the form of a rotating target anode, causes deceleration of the electrons thereby producing x-rays. The cathode cup for the cathode assembly can be used to hold the filament to allow electrical current to be supplied thereto. The cathode cup can also act to focus the emitted electrons towards a focal spot on the focal track of the anode target.
Cathode cups presently used in x-ray tubes are typically manufactured from a TZM (titanium-0.5%, zirconium-0.008%, balance molybdenum) alloy. In addition, nickel and non-nickel alloys can be used for the cathode cup in certain metal-framed x-ray tube applications.
A cathode cup is exposed to, and typically absorbs, a portion of the heat generated by the filament as well as the anode focal track, due at least in part to the high temperatures to which the filament and focal track are heated and the proximity of the cathode cup to the hot filament and focal track. The high temperatures to which cathode cups are exposed and the rapid thermal cycling of the cathode may cause a number of problems. In order to withstand such high temperatures resulting from the absorption of radiated heat from the filament and focal track, a cathode cup typically needs be comprised of a material capable of withstanding such temperatures, such as refractory alloys. High cup temperatures can lead to creep deformation of various components of the cathode cup assembly and cathode arm. The high cup temperatures can also lead to increased thermal mismatch and distortion of the joints between the cathode cup and arm, and/or the filament support leads and cup. All of the foregoing may lead to undesirable drift and/or distortion of the focal spot, which are undesirable.
During x-ray tube operation, the cathode cup is heated primarily via absoption of radiation from the incandescent filament. The cathode cup may also be heated, to a lesser extent by the anode focal track. In operation, the filament may become heated to temperature greater than about 2500.degree. C. Although the cathode cup is not in direct contact with the filament, and the filament may be cycled off and on during operation, the cathode cup temperature can rise to about 600.degree. C.
It has been proposed to provide the interior surface of cathode caps in "fast warm-up" cathode ray tubes for televisions with a black or dark gray (non-reflective coating. The coating may increase the rate at which the cathode cap is heated to operating temperatures so it can emit electrons. For example U.S. Pat. No. 3,958,146 ('146) discloses a "fast warm-up" cathode cap for use in a cathode ray tube. In '146, a heater is disposed within the nickel alloy cathode cap, and the cap has an electron emissive material on the outer surface of the closed end for emitting elections. The inner surface is clad with a nichrome material. When the cap and nickel-chromium material covering the inner surface thereof is exposed for about 10 minutes or longer in wet dissociated ammonia at a temperature in a range from about 900.degree. C. to about 1300.degree. C. Thus, the inner surface of the cap oxidizes to form a dark gray or black surface. In operation, the black surface increases the rate at which heat is absorbed into the cap from the heater, thus the rate at which the cap is heated to operating temperatures at which electron emission will occur increases.
U.S. Pat. No. 4,673,842 to Grieger et al., commonly assigned to General Electric Corporation, discloses a cathode cup having a base formed of TZM, and an exposed upper surface of graphite, which is bonded to the base. The graphite upper surface of the cathode cup is coated with pyrolitic carbon or a silicon carbide graphite composition. The pyrolitic carbon or a silicon carbide graphite composition is non-infrared reflective and minimizes dust. It can also eliminate possible welding of the filament to the cathode cup in the event of contact therebetween.
Therefore, a need exists to provide cathode cup structures that overcome the above-noted deficiencies. Further, a need exists to provide enhanced cathode cups for x-ray tubes.